The point of a mirror universe storyline is to show the regular cast in a completely different light. In that, 'Yesterday's Enterprise' succeeded, even if it wasn't a true mirrorverse episode. Voyager did something similar with 'Living Witness' where everyone put on their black hats for an episode and had fun with it. DS9 did return us to the mirrorverse, but did it in a weird way - the terrans were now the underdogs, the klingons and cardassians were still the bad guys, and the entire thing was treated more like a divergent timeline as opposed to the way TOS treated it.

The point of a mirror universe storyline is to show the regular cast in a completely different light. In that, 'Yesterday's Enterprise' succeeded, even if it wasn't a true mirrorverse episode. Voyager did something similar with 'Living Witness' where everyone put on their black hats for an episode and had fun with it. DS9 did return us to the mirrorverse, but did it in a weird way - the terrans were now the underdogs, the klingons and cardassians were still the bad guys, and the entire thing was treated more like a divergent timeline as opposed to the way TOS treated it.